You ask for my opinion of Dr. Robert Gentry's work on
pleochroic polonium halos. I spent a number of hours reviewing
this fascinating work with him some weeks ago. I was impressed
with the clarity of the evidence for "anomalous halos"—that is,
cases where there are rings indicating the presence of some
members of the normal radioactive decay chain without the other
members of the family tree that normally are present, that
normally do show up in rings of their own, and that have to be
there on present views of the radioactive decay chains involved. If
the evidence is impressive, the explanation for it is far from clear.
I would look in normal geologic process of transfer of materials
by heating and cooling; in isomeric nuclear transitions; and in
every other standard physical phenomenon before I would even
venture to consider cosmological explanations, let alone radical
cosmological explanations. To explore all the avenues that need
exploring would take months, not the few hours I was privileged
to spend in Dr. Gentry's company. A few days ago I reviewed this
work, all too briefly, with Dr. G. Wasserburg of Cal Tech, who is
an expert in the radioactive dating of rocks, whose opinion would
be much more to the point than mine, especially if he will give it
to you in writing.*

JOHN A. WHEELER
(Professor of Physics,
Princeton University)

*Professor Wheeler requested that his letter be printed in full.
Dr. Wasserburg's views have not been obtained.

THE CONSERVATISM OF SCIENCE

Many have noted a conservatism in science essential to its orderly
advance: skepticism toward radically new ideas enables scientific
journals to retain focus, prevents anarchic descent into theoretical
chaos, and makes it possible to extend currently reigning theories as
far as they can bear before replacing them with other theories yet
more embracive. A successfully modified, "tested" theory is
preferable to a new "untried" theory. And so scientific knowledge
advances in an orderly fashion, with as few wrong turns as possible.*

[* This conservatism—and its deceptive advantages—will receive continuing
discussion in these newsletters.]

Gentry has so far avoided clashing with this conservatism, chiefly
by concentrating his efforts on publication of data rather than
discussion of their implications—and also by the good fortune that
his work has been slow to draw widespread attention. That is
beginning to change, however. But perhaps the reaction of a number
of prominent physicists to Gentry's work on polonium halos (see
insets on this and the following page) is the most significant gauge of
what will be forthcoming. This reaction is noteworthy both for the
confidence expressed in Gentry's work and for the almost uniformly
conservative—albeit open—stance toward any extrapolations from
the raw data that challenge accepted theory. Of those whose opinions
we sampled, only one seemed to suggest (without wishing to be
quoted) that we not publicize Gentry's work. He felt that the subject
should be "left to the experts," while cautioning that it is too early to
reject the conventional view of Earth's history.

In the end, it is, presumably, the evidence which will decide the
issue. Let us look more closely at the radiohalos themselves.

THE NATURE OF HALOS

If a small grain (inclusion) containing radioactive atoms is
embedded in certain rock minerals, the alpha particles emitted from
the radioactive atoms travel outward from the inclusion and damage
the crystalline structure of the mineral, in time producing the visible
discoloration typifying halos. Since each type of radioactive atom
emits alpha particles with a characteristic energy, and since this
energy determines how far the particle will travel in the host mineral,
the diameter of a halo's rings guides researchers in determining which
radioactive element is responsible for the halo. If the radioactive
element in an inclusion is the initiator of a decay series, then a group
of concentric halo rings results, each ring corresponding to a step in
the decay series, that is, to alpha particles of a particular energy. In
the case of the 238U series, with eight alpha-decay steps, there are five
distinct halo rings (some of the alpha particles are so close together in
energy that their rings are not distinguishable).

The conventional argument drawn from observed radiohalo sizes
is summarized by Struve:

"There is excellent evidence that the rates of radioactive processes
measured in the laboratory at the present time are valid also for the
remote past. If a radioactive element and its decay products are embedded
in a crystal, each alpha particle emitted during disintegration
travels a certain distance that depends only on the rate of that particular
decay step. The more rapid this rate, the greater the energy of the alpha
particles, and the farther they go before being stopped and producing a
color change in the crystal.

A uranium-238 halo (left) and a polonium-210 halo in biotite.
Scale is 1 cm equivalent to 45 μm [in the original publication, ed.].

"Suppose a speck of 238U has remained undisturbed since the
formation of a mineral containing it. Then, because the rate of disintegration
at each successive emission is different, eight concentric rings of
mineral discoloration will be found surrounding the particle of uranium.
These rings . . . have been found in many rocks of different geological
ages, and the diameters of the respective rings are always the same.

"Thus it can be concluded that the rates of disintegration of
uranium and thorium are constant" (2).

As we will learn in a subsequent review, the evidence from halos
has led Gentry in a direction quite opposite from Struve's. But more
than that, Gentry's halo research appears to strike at the roots of
virtually all contemporary cosmologies, posing a fundamental
problem which has so far resisted every effort to solve it in
conventional terms. This is the problem of the polonium halos.